Technical Field of the Disclosure
The present disclosure relates in general to reliable control of high rotor pole switched reluctance machine (HRSRM), and more particularly to a system and method for eliminating the use of position sensors in HRSRM which improve the accuracy of rotor position estimation utilizing a combination of self-inductance and mutual-inductance values.
Description of the Related Art
A wide variety of methods have been developed to provide optimal control strategies for switched reluctance machines (SRMs). Compared to conventional induction and synchronous motor drive systems, SRM drives are relatively simple in construction, offer wide speed range capabilities and are economic to manufacture. Further, because of the absence of windings and permanent magnets on the rotor they are attractive for robust and harsh environment applications. In addition, the converter, which applies power to the SRM drive, often requires fewer power devices and, therefore, is more economical and reliable. Building on these advantages, SRM drive systems provide an advanced alternative to conventional drive systems in several variable speed drive and industrial applications. SRM drives conventionally have multiple poles on both the stator and rotor. The stator includes phase windings, but the rotor does not include windings or magnets.
In an SRM system, the stator generates torque on the rotor when the current passing through each phase winding is switched on in a predetermined sequence. By properly positioning the firing pulses relative to the rotor angle, forward or reverse operations may be obtained. Usually, the desired phase current commutation is achieved by feeding back a rotor position signal to a controller from a shaft position sensor, e.g., an encoder or resolver. For economic reasons in small drives and reliability reasons in larger drives and to reduce size, weight, and inertia in all such drives, it is desirable to eliminate this shaft position sensor. In order to overcome this shortcoming, a new sensorless technique for high rotor pole switched reluctance machine (HRSRM) has been introduced.
Compared to a conventional SRM, the HRSRM exhibits higher static torque capabilities, which effectively addresses torque ripple and acoustic noise problems. The design parameters of the power converters are different in HRSRMs vs. HRMs. This is because the HRSRM has a different inductance profile and a higher number of strokes. Most reliable techniques for conventional HRSRM operation utilize the self-inductance of the phase coil to estimate position. The HRSRM has a higher number of rotor poles for the same circumference as a conventional SRM. The higher number of rotor poles reduces the angular travel per excitation. HRSRM has shown an approximate increase of 83% in static torque capabilities as compared to a 6/4 SRM under steady state operations for the same joule losses. However, the larger number of rotor poles leads to a smaller gap and the arc length (or angular length) between two rotor poles is smaller. Consequently, unaligned inductance of the machine is lower and the resultant the self-inductance profile for the HRSRM tends to become flatter, which leads to unreliable position estimations. Thus, the use of self-inductance of the phase coil alone is often not sufficient to estimate the accurate rotor position in the HRSRM.
Several methods have been developed to solve the above shortcomings. One of the existing reliable control methods includes a technique for measuring mutual inductance. In a first example embodiment of this technique, a voltage pulse is applied to the primary coil when the machine is stationary. By measuring current in the primary coil and measuring induced voltages in adjacent open circuited coils, mutual inductance may be determined. In another example embodiment, a voltage pulse is applied to the primary coil when the machine is stationary. The secondary coil is allowed to free-wheel current through the phase. By measuring the time taken by the primary phase to reach a peak or preset threshold value, the mutual inductance for the known position of a rotor may be determined. However, this technique usually does not provide an accurate estimation of rotor position since this method only utilizes mutual inductance for the rotor position estimation.
Some other reliable control methods include a controller that implements a model of at least one active phase representing dynamic magnetic machine characteristics. The controller determines machine control signals based on rotational position obtained by numerically solving the model with measured machine operating parameters. The model may be implemented as the sum of orthogonal functions relating active phase voltage and current with constants derived from phase inductance to obtain the rotor angle. Yet another reliable control method includes probing a selected diagnostic phase with a pulse injection process; measuring an actual operating characteristic of the SRM; computing an inductance based on the actual operating characteristic and correlating the inductance with a position to formulate an estimated position; modeling the SRM to formulate an observer-based estimated position; selecting one of the estimated positions, the observer-based estimated position, and a combination thereof to formulate a selected position of the SRM; and controlling said SRM based on said selected position and a command. However, in most of these methods, while evaluating the machine performance, the mutual inductances between phases are neglected, resulting in an unreliable position estimation. Further, these methods do not provide an accurate rotor position in a HRSRM configuration.
Therefore, there is a need for a method of reliable control of a HRSRM using a combination of self-inductance and mutual inductance to enhance the accuracy of rotor position estimation. The method would use only terminal measurements such as, voltages, currents and time and would not require additional hardware or memory. Further, the method would be able to accurately estimate instantaneous rotor position in HRSRM and SRM, irrespective of motor speed or direction, and without resorting to a rotor position sensor. Finally, the method would be reliable, robust and preferably cost effective. The present embodiment accomplishes these objectives.